Today it is common to use renewable energy for power generation, including wind energy as one of the most efficient. Wind energy can generate electricity from wind by means of wind turbines. Said wind turbines basically consist of a tower, a nacelle that houses the electrical generator, a rotor formed in turn by at least two blades, and a power train that transmits power from the rotor to the electric generator. The power train may comprise a gear box with a low speed shaft connected to the rotor and a high speed shaft connected to the electric generator.
In multi-megawatt wind turbines, there is a trend towards larger rotors, which provide energy at a lower cost. In these configurations, there is a growing importance of the control system. This system maximizes energy production while limiting the mechanical loads produced by the wind. For this, the control system acts on the blade pitch angle and the torque required from the generator.
Firstly, the pitch angle is controlled by actuators arranged in the root of each blade, making the blade rotate around its longitudinal axis. Such actuation varies the aerodynamic performance of the blade, while on the other hand, the control system modulates the torque required to the generator from the converter.
The energy production of a wind turbine under different wind conditions begins above a certain wind speed commonly known as Vcut-in, so that with higher winds the turbine starts to rotate producing energy, and with lower winds the turbine remains paused with a safety pitch angle of 90°, which causes the rotational speed of the rotor to be substantially zero. There is also a wind speed for which the wind turbine reaches the rated power Vrated.
The graph of the required electric torque T vs the rotational speed of the electric generator ω, shows a variable speed section in which a control system of the prior art determines the required electric torque T as a function of the generator rotational speed ω, in order to maintain the blade tip speed ratio (Tip speed ratio or λ) constant at an optimum which maximizes the capture of aerodynamic wind power.
λ=(ω×R)/v, wherein
ω: Rotor rotational speed
R: radius of the rotor
v: incident wind speed
To perform the above control to keep the tip speed ratio constant to an optimal value that maximizes the capture of aerodynamic wind power, there are prior patents that propose a closed loop control of λ acting on the torque of the generator.
It is the case of the international patent application WO2008119994A2, which describes a controller that modifies the rotational speed of the rotor by acting on the electrical torque depending on the local wind speed measured to maintain the tip speed ratio within predetermined limits. If for instance the maximum energy efficiency is given for a λ=3.5, the controller is programmed to maintain the tip speed ratio between 3.5 and 4.5 (default or optimum value). An anemometer measures instantaneous wind speed at the frequency of 2 to 4 Hz and this is sent to the controller, which calculates the instant λ respect to the default or optimum value.
However, it is not ideal to use directly a measure of λ, as it requires a measure of the wind speed with an anemometer placed in the nacelle, which is a very noisy signal and sensitive to environmental effects such as wind shear, up flow, etc. In addition, the usual location of the anemometer at the rear of the nacelle means that its measurement is disturbed by the rotor.
To avoid this drawback, it is usual to perform a control of the rotational speed of the wind turbine by acting on the electrical torque so that the wind turbine operates in the variable speed area according to the T/ω ratio established.
Due to various turbine or environmental related effects, the machine can go to work in non-optimal conditions, either from the point of view of energy production or the structural integrity of the machine, so that the power generated by the wind turbine for a given wind speed is less than that which would ideally be produced with the same wind speed.
The effects related to the wind turbine include:                Misalignment with respect to the wind direction. This may be due to assembly errors of the direction sensor (vane), wind flow distortion in the vane caused by the rotor, etc. . . . .        Dirt, ice or blade deterioration. It reduces the efficiency of the turbine.        Component degradation. This decreases component performance affecting the overall efficiency of the machine.        
The effects related to the environment of the machine that affect its performance include:                Variations in density.        Up flow.        Shear profile.        
In the prior art, the control parameters of the control system of wind turbines are calculated theoretically or with the help of simulation tools. In addition, methods for the detection of anomalous conditions are based on the comparison of the generated power with a given wind and the theoretical power that should be generated with that wind. These methods have the disadvantage that, for comparisons to be valid, only data corresponding to wind speeds included in relatively small wind intervals (0.5 m/s−1 m/s) may be considered.
Both in the case of the international application WO2008119994A2 and in the patent with publication number CA1245283A1, which describes a wind energy conversion system with a closed loop control system based on an error signal which is a measurement of the difference between a desired value or reference value of the tip speed ratio (λopt) and λ associated with when the wind turbine receives a gust of wind, when the rotational speed is changed based on the comparison of instantaneous values of the tip speed ratio with the optimum tip speed ratio, these controls assume that the instantaneous differences in λ are due to the fact that the wind speed has changed, for example due to a gust of wind, so it is necessary to change the rotational speed to be in the optimal area. However, as explained, there are environmental conditions or of the turbine itself that make it operate outside its point of maximum performance, which are not corrected by changing the rotational speed of the rotor.
The present invention relates to a control method for detecting situations in which the machine is not working at its optimum operating point overcoming the disadvantages of the aforementioned prior art cases.
Also, once these situations have been detected, the control method of the present invention allows to perform an automatic correction of the control parameters and return the wind turbine to its optimum operating point.